Day in the Life of a Functional Programmer

Transcript

1. A Day in the Life of a Func onal
   Programmer
   Richard Dallaway
   

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2. Modern Development
   Func onal Programming with Types
   

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3. Why now?
   

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4. Why now?
   Modern compilers have powerful type
   system
   

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5. A Day in the Life
   

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6. Func onal Programming
   Is For Everyone
   Makes Change Easier
   

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7. The Morning Standup
   

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8. CRM
   RDBMS
   REST Server
   

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9. $ http GET :8080/customers
   HTTP/1.1 200 OK
   Content-Type: application/json
   [
   {
   "id": 1,
   "name": "Alice",
   "phone": "+1 555 1234"
   },
   {
   "id": 2,
   "name": "Bob",
   "phone": "+1 555 5678"
   }
   ]
   

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10. As a customer, I should have a
    subscrip on, so I can receive the
    company magazine.
    

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11. No subscrip on
    Ac ve subscrip on
    Lapsed subscrip on
    

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12. Algebraic Data Types
    Structural Recusion
    Unfamiliar names for simple powerful ideas
    

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13. sealed trait Subscription
    

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14. sealed trait Subscription
    case object Never extends Subscription
    case object Active extends Subscription
    case object Lapsed extends Subscription
    

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15. sealed trait Subscription
    case object Never extends Subscription
    case class Active(expires: Date) extends Subscription
    case object Lapsed extends Subscription
    

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16. sealed trait Subscription
    case object Never extends Subscription
    case class Active(expires: Date) extends Subscription
    case object Lapsed extends Subscription
    case class Customer(
    name : String,
    phone : String,
    sub : Subscription
    )
    

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17. sealed trait Subscription
    case object Never extends Subscription
    case class Active(expires: Date) extends Subscription
    case object Lapsed extends Subscription
    case class Customer(
    name : String,
    phone : String,
    sub : Subscription
    )
    def transition(sub: Subscription, at: Date): Subscription = ???
    

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18. sealed trait Subscription
    case object Never extends Subscription
    case class Active(expires: Date) extends Subscription
    case object Lapsed extends Subscription
    case class Customer(
    name : String,
    phone : String,
    sub : Subscription
    )
    def transition(sub: Subscription, at: Date): Subscription =
    sub match {
    case Lapsed | Never => sub
    case Active(exp) => ???
    }
    

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19. sealed trait Subscription
    case object Never extends Subscription
    case class Active(expires: Date) extends Subscription
    case object Lapsed extends Subscription
    case class Customer(
    name : String,
    phone : String,
    sub : Subscription
    )
    def transition(sub: Subscription, at: Date): Subscription =
    sub match {
    case Lapsed | Never => sub
    case Active(exp) =>
    if (at > exp) Lapsed else sub
    }
    

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20. “Marke ng just reminded me we
    give free subscrip ons to some
    customers”
    — Product Owner
    

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21. sealed trait Subscription
    case object Never extends Subscription
    case class Active(expires: Date) extends Subscription
    case object Lapsed extends Subscription
    

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22. sealed trait Subscription
    case object Never extends Subscription
    case class Active(expires: Date) extends Subscription
    case object Lapsed extends Subscription
    case object Freebie extends Subscription
    def transition(sub: Subscription, at: Date): Subscription =
    sub match {
    case Lapsed | Never => sub
    case Active(exp) =>
    if (at > exp) Lapsed else sub
    }
    ERROR: non‐exhaus ve match
    

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23. Move side effects out
    

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24. def transition(c: Customer, at: Date): Action = ???
    

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25. def transition(c: Customer, at: Date): Action = ???
    sealed trait Action
    case object NoAction extends Action
    case class EncourageRenewal(c: Customer) extends Action
    case class Expire(c: Customer) extends Action
    ...
    

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26. def transition(c: Customer, at: Date): Action = ???
    sealed trait Action
    case object NoAction extends Action
    case class EncourageRenewal(c: Customer) extends Action
    case class Expire(c: Customer) extends Action
    ...
    def perform(a: Action): Unit =
    a match {
    case NoAction =>
    case Expire(c) => sendSMS(c.phone)
    ...etc
    }
    

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27. ADTs
    Safe
    Versi le
    Separate data & behaviour
    

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28. Output JSON to the Client
    {
    "id": 1,
    "name": "Alice",
    "phone": "+1 555 1234",
    "sub": {
    "type": "expired"
    }
    }
    

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29. Type Class
    Big things from ny building blocks
    

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30. Turn Data into JSON text
    7.toString
    

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31. Turn Data into JSON text
    7.toString
    7
    "\"" + "Hello" + "\""
    "Hello"
    

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32. Turn Data into JSON text
    7.toString
    7
    "\"" + "Hello" + "\""
    "Hello"
    { greeting: "Hello", count: 7 }
    

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33. Turn Data into JSON text
    7.toString
    7
    "\"" + "Hello" + "\""
    "Hello"
    { greeting: "Hello", count: 7 }
    Int => String
    String => String
    

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34. Turn Data into JSON text
    7.toString
    7
    "\"" + "Hello" + "\""
    "Hello"
    { greeting: "Hello", count: 7 }
    Int => String
    String => String
    T => String
    

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35. trait JsonFormat[T] {
    def format(value: T): String
    }
    

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36. trait JsonFormat[T] {
    def format(value: T): String
    }
    val intFormat = new JsonFormat[Int] {
    def format(value: Int): String = value.toString
    }
    val strFormat = new JsonFormat[String] {
    def format(value: String): String =
    "\"" + value + "\""
    }
    

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37. trait JsonFormat[T] {
    def format(value: T): String
    }
    val intFormat = new JsonFormat[Int] {
    def format(value: Int): String = value.toString
    }
    val strFormat = new JsonFormat[String] {
    def format(value: String): String =
    "\"" + value + "\""
    }
    def outputJson[T](value: T)(jf: JsonFormat[T]) =
    jf.format(value)
    outputJson(7)(intFormat)
    // 7
    

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38. trait JsonFormat[T] {
    def format(value: T): String
    }
    implicit val intFormat = new JsonFormat[Int] {
    def format(value: Int): String = value.toString
    }
    implicit val strFormat = new JsonFormat[String] {
    def format(value: String): String =
    "\"" + value + "\""
    }
    def outputJson[T](value: T)(implicit jf: JsonFormat[T]) =
    jf.format(value)
    outputJson(7)
    // 7
    

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39. [ "Hello", "there" ]
    [ 1, 2, 3, 4 ]
    [ [1,2], [3,4] ]
    

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40. implicit def listFormat[T] =
    new JsonFormat[List[T]] {
    def format(values: List[T]): String = {
    ???
    }
    }
    

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41. implicit def listFormat[T] =
    new JsonFormat[List[T]] {
    def format(values: List[T]): String = {
    val formattedValues = {
    ???
    }
    "[" + formattedValues.mkString(",") + "]"
    }
    }
    

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42. implicit def listFormat[T] =
    new JsonFormat[List[T]] {
    def format(values: List[T]): String = {
    val formattedValues = for {
    v <- values
    } yield ???
    "[" + formattedValues.mkString(",") + "]"
    }
    }
    

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43. implicit def listFormat[T](implicit jf: JsonFormat[T]) =
    new JsonFormat[List[T]] {
    def format(values: List[T]): String = {
    val formattedValues = for {
    v <- values
    } yield jf.format(v)
    "[" + formattedValues.mkString(",") + "]"
    }
    }
    

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44. implicit def listFormat[T](implicit jf: JsonFormat[T]) =
    new JsonFormat[List[T]] {
    def format(values: List[T]): String = {
    val formattedValues = for {
    v <- values
    } yield jf.format(v)
    "[" + formattedValues.mkString(",") + "]"
    }
    }
    def outputJson[T](value: T)(implicit jf: JsonFormat[T]) =
    jf.format(value)
    outputJson( List(1,2,3,4) )
    [1,2,3,4]
    

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45. implicit def listFormat[T](implicit jf: JsonFormat[T]) =
    new JsonFormat[List[T]] {
    def format(values: List[T]): String = {
    val formattedValues = for {
    v <- values
    } yield jf.format(v)
    "[" + formattedValues.mkString(",") + "]"
    }
    }
    def outputJson[T](value: T)(implicit jf: JsonFormat[T]) =
    jf.format(value)
    outputJson( List(1,2,3,4) )
    [1,2,3,4]
    outputJson( List( List(1,2), List(3,4) ) )
    [ [1,2], [3,4] ]
    

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46. Type Class
    Small Building Blocks
    Combined by the Compiler
    Used Everywhere
    

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47. case class Customer(
    name : String,
    phone : String,
    id : Long
    )
    sql" select name, phone, id from customer ".query[Customer]
    

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48. case class Customer(
    name : String,
    phone : String,
    id : Long
    )
    sql" select name, phone, id from customer ".query[Customer]
    implicit val SubscriptionMeta: Meta[Subscription] =
    Meta[String].nxmap(ch =>
    ch match {
    case "A" => Active
    case "L" => Lapsed
    case "N" => Never
    }, sub =>
    sub match {
    case Active => "A"
    case Lapsed => "L"
    case Never => "N"
    })
    

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49. Used Everywhere
    ADTs
    Type Classes
    

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50. As a customer, I should have an
    control over my details, so I can
    update some or all of them.
    

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51. class CustomerAPI(db: Database) {
    val service = HttpService {
    case GET -> Root / "customers" =>
    Ok(db.list)
    case GET -> Root / "customers" / IntVar(id) =>
    db.find(id).flatMap {
    case Some(customer) => Ok(customer)
    case None => NotFound()
    }
    }
    }
    

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52. class CustomerAPI(db: Database) {
    val service = HttpService {
    case GET -> Root / "customers" =>
    Ok(db.list)
    case GET -> Root / "customers" / IntVar(id) =>
    db.find(id).flatMap {
    case Some(customer) => Ok(customer)
    case None => NotFound()
    }
    case PATCH -> Root / "customers" / IntVar(id) =>
    decode the patch
    lookup the customer in the database
    found a record?
    somehow apply the patch to the record
    update the database
    otherwise, 404 the request
    }
    }
    

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53. Simplified Patch
    {
    "name" : "Robert"
    }
    or
    {
    "name" : "Robert",
    "phone : "+1 555 555"
    }
    

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54. Simplified Patch
    (Customer, Patch) => Customer
    

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55. case class Customer(
    id : Long,
    name : String,
    phone : String
    )
    case class CustomerPatch(
    name : Option[String],
    phone : Option[String]
    )
    

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56. case class Customer(
    id : Long,
    name : String,
    phone : String
    )
    case class CustomerPatch(
    name : Option[String],
    phone : Option[String]
    )
    trait Merge[Into, From] {
    def merge(i: Into, f: From): Into
    }
    def merge[Into, From](i: Into, f: From) = ???
    

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57. case class Customer(
    id : Long,
    name : String,
    phone : String
    )
    case class CustomerPatch(
    name : Option[String],
    phone : Option[String]
    )
    trait Merge[Into, From] {
    def merge(i: Into, f: From): Into
    }
    def merge[Into, From](i: Into, f: From)
    (implicit m: Merge[Into, From]) = m.merge(i, f)
    

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58. case class Customer(
    id : Long,
    name : String,
    phone : String
    )
    case class CustomerPatch(
    name : Option[String],
    phone : Option[String]
    )
    trait Merge[Into, From] {
    def merge(i: Into, f: From): Into
    }
    def merge[Into, From](i: Into, f: From)
    (implicit m: Merge[Into, From]) = m.merge(i, f)
    implicit def customerMerge: Merge[Customer, CustomerPatch] =
    ???
    

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59. case class Customer(
    id : Long,
    name : String,
    phone : String
    )
    case class CustomerPatch(
    name : Option[String],
    phone : Option[String]
    )
    trait Merge[Into, From] {
    def merge(i: Into, f: From): Into
    }
    def merge[Into, From](i: Into, f: From)
    (implicit m: Merge[Into, From]) = m.merge(i, f)
    implicit def customerMerge: Merge[Customer, CustomerPatch] =
    ???
    implicit def optMerge = new Merge[String, Option[String]] {
    def merge(initial: String, update: Option[String]) = update match {
    case Some(value) => value
    case None => initial
    }
    }
    

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60. case class Customer(
    id : Long,
    name : String,
    phone : String
    )
    case class CustomerPatch(
    name : Option[String],
    phone : Option[String]
    )
    trait Merge[Into, From] {
    def merge(i: Into, f: From): Into
    }
    def merge[Into, From](i: Into, f: From)
    (implicit m: Merge[Into, From]) = m.merge(i, f)
    implicit def customerMerge: Merge[Customer, CustomerPatch] =
    ???
    merge(
    Customer(1, "Bob", "555-123"),
    CustomerPatch(Some("Robert"), None)
    )
    Customer(1, "Robert", "555-123")
    

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61. Pu ng it all together...
    case req @ PATCH -> Root / "customers" / IntVar(id) =>
    req.decode[CustomerPatch] { patch =>
    db.find(id).flatMap {
    case None => NotFound()
    case Some(customer) =>
    val updated = merge(customer, patch)
    Ok(db.update(updated))
    }
    }
    val merge = bulletin.AutoMerge[Customer, CustomerPatch]
    case class CustomerPatch(
    name : Option[String],
    phone : Option[String]
    )
    

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62. ADTs
    Safe, Versi le
    Type Classes
    Combining Small Building Blocks
    

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63. Scala
    Modern Powerful Compiler
    Func onal Programming
    JVM, Scala Na ve, Scala.js
    

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64. Help Make Change Easier
    

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65. Thank You
    h ps:/
    /underscore.io/
    

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